you tell him the truth, that it is a sin becuase it is of selfish desire that goes against one, God first commandment to his people, to multiply and overcome the earth. no life can come of the homosexual act.
two, it is forbidden in Word of God, see leviticus and read about soddam and gomorah.
the key to telling the truth is, that you must tell it with the love and compassion of Christ. How would Jesus tell it? well he would just tell it, but there would be no mistaking the love in his eyes when He told you.
All of this hinges on your friends belief, faith, and willingness to serve the Lord.
**scientifically, it is naturally wrong, going against the laws of nature, and it has never been proven to be a genetic condition. He has to be willing to do this research as well, for then it shows he has real interest in the truth. **
It is not sinful to be a “homosexul” but it is a sin to practice it. It is his fight, as we all have our fights, some agains substances, some against Lust, gluttony, greed, pride…etc…
one is not worse than the other, but in his case, if he wants to serve the Lord and Obey Him, he must put this on the alter and remain chaste, if he doesn’t think he could work towards being attracted to women, which is possible, there are many support groups of “ex” gays that have made the transition and started families.
Peace of the Lord be with you.